Wind turbine coupling to mitigate torque reversals

ABSTRACT

A wind turbine power generating system, including a wind turbine connected to a speed-increasing gearbox having an output shaft. An electrical generator having an input shaft is also provided. A coupling interconnects the input and output shafts. The coupling includes a high torsional wind-up and/or displacement ability in parallel with a high frictional slip ability, such that during normal operation there is little or no frictional slippage and during a transient torque reversal the loads in the turbine drive system are decreased, thus decreasing the impact loads on the gearbox bearings.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of pending U.S. provisionalapplication Ser. No. 61/882,856, filed Sep. 26, 2013, by the same title,which is fully incorporated herein by reference.

TECHNICAL FIELD

The invention herein relates to couplings interposed between windturbines and electrical generators. Particularly, the invention relatesto such a coupling configured to dissipate the adverse effects of torquereversals on gearbox bearings in a wind turbine generator system.

BACKGROUND ART

Well over 100,000 megawatt and multi-megawatt wind turbines have beeninstalled over the past decade, almost all using a similar drive systemincorporating a gearbox as a speed increaser, positioned between theturbine blades and the generator. The gearboxes are designed for 20-yearlife, but typically need repair or replacement in 5 to 10 years or less.Axial cracking of gearbox bearings is becoming a major cost factor inthe return on investment of wind farms. Impact loading during transienttorque reversals has been recognized as a root cause of this damage.Recently research has shown that an unusual mode of bearing damagecalled White Etch Area (WEA) damage is causing the axial cracking of thebearings. WEA damage is actually a microscopic material alteration thatcreates super-hard inclusions like slivers just below the bearingraceway where cracks can initiate and grow. Severe and rapid microscopicplastic deformation is suspected as the cause of WEA damage.

During a torque reversal, the load zone of the gearbox bearings suddenlyshifts 180 degrees. The bearing rollers radially impact onto the racewayalong with a simultaneous high axial load reversal from the helicalgears. Both the magnitude and the rate of the impact loads and axialsurface traction loads determine the potential for WEA plasticdeformation in the bearing inner race. The higher the torsional naturalfrequency of the drive train's spring mass system, the greater thetorque rate of change, and thus the higher the strain rate as therollers impact the bearing inner race . As wind turbines have increasedin size, the high strain rate during rapid bearing load zone reversals,along with high impact stress, appears to be exceeding a threshold whereWEA damage is initiated in the bearing inner raceway. Once initiated thenormal roller loading can cause axial cracking and bearing failures inas little as a year or two.

In a wind turbine generator system, high inertia characterizes theentire system, from the turbine blades, main shaft, gearbox high-speedcoupling and into the generator itself. Indeed, the highest inertia istypically at the opposite ends of the system—at the blades and thegenerator. On torque reversal, the high inertia of the system cansignificantly impact all of the system components, and particularly thegearbox. The asymmetric torque-limiting clutch system described in U.S.Patent Application Publication US 2012/045335 A1 describes a solution tothis problem. An alternate solution contemplates increasing thetorsional wind-up of the system (including certain of, blades, a mainshaft, a gearbox, a high-speed shaft/coupling and a generator) whichwould lower the natural frequency. If this were done alone, it couldcause other problems in the turbine drive system; such as resonantfrequency issues in other parts of the turbine. For instance, it isknown that the coupling spacer between the gearbox and the generator canhave a problematic axial natural frequency that can cause spacer elementresonance and destruction. Any changes to the system natural frequencyduring normal operation may necessitate a recertification of theturbine.

Increasing the torsional wind-up must be done in a way that does notaffect normal operation of the turbine. This could be accomplished witha high frictional slip ability in parallel with a high torsional wind-upand/or displacement ability. For example, if the frictional torquesetting was at 40% of the rated turbine torque, there would be noslippage during normal operation between 20% and 100% of the ratedturbine torque as apparent from FIG. 1. The only time the frictionslippage would occur is when the drive system sees a total torquevariation exceeding 80% of the rated turbine torque, for example forbrief periods during startups and shutdowns. Significant slippage wouldonly occur during rare system transient torque reversals exceeding thefrictional slip setting. It is contemplated that the frictional torquesetting should be such as to accommodate some small slippage duringnormal startup and shutdown operation to keep the friction surfacesclean and free of corrosion.

If the high torsional wind-up is effected by a torsional spring, ascontemplated in an embodiment of the invention, the torsional springrate may be asymmetric so that the spring rate in reverse could be loweror near zero for a portion of the displacement. Any reverse torqueevents would slip at frictional resistance only of say 40% of normalturbine torque. The reverse angle of travel would need to be sufficientto absorb reverse transient wind-up energy of the drive system. This mayrequire a torsional movement of 10 to 50 degrees or greater for typicalturbines with generators operating at 1000 rpm or more. For turbineswith lower generator operating speeds, the required torsionaldisplacement would be lower, in the range of 1 to 5 degrees per 100 rpm.

The typical coupling systems of existing wind turbines are designed withsignificant parallel, angular and axial shaft misalignment capabilitybetween the gearbox and the generator in order to accommodate theflexing of the lightweight base plate structure. These coupling systemstypically have zero backlash and are torsionally very rigid with verylittle wind-up ability. The torsional characteristics are criticallyimportant to preventing resonant vibration problems in the drive systemand turbine components. Some coupling systems are equipped withfrictional torque limiters set at 150 to 200% of the rated turbinetorque. They are intended to protect the coupling from the very hightorque overloads such as generator short circuits. These torque limitershave proved to be ineffective in protecting the drive system andespecially the gearbox from transient torque reversals whose impactloads on the gearbox bearings can dramatically shorten life.

Coupling systems that utilize torsional wind-up in parallel with lowfrictional damping, such as Spaetgens U.S. Pat. No. 2,909,911 and LechU.S. Pat. No. 4,5548,311, have been around for a long time. They aregenerally used on internal combustion engines. Their torsional wind-upability is used to tune the natural frequencies of the system to beoutside the operating range of the equipment. Their frictional dampingcomponent that is in parallel with the torsional wind-up is typicallyvery small and is used to control clutch plate and gear rattle noise anddamage during idling and shifting. These types of couplings generallyare integrated with the engine clutch whose frictional slip setting isvery high and is in series with the torsional wind-up ability, not inparallel. Lech is a good example. The frictional component that is inparallel has a very low frictional slip setting.

A key to the present invention's success is a coupling system with ahigh torsional wind-up and/or displacement ability, along with a highfrictional slip ability to dampen the system significantly only during atransient torque reversal event (see FIG. 1). A typical turbine with ahigh speed generator operating at 1000 to 1800 rpm would require atleast 10 degrees of reverse slippage with a torque setting of at least10% of turbine rated torque. Ideally, the reverse slippage would exceed20 degrees at 40% reverse torque. Nowhere in the prior art is there adrive system with such a combination of torsional displacement and/orwind-up with torsional frictional damping capable of taming high torquereversals. This is certainly not true for the uniquely challengingreversals of wind turbines.

SUMMARY OF THE INVENTION

In light of the foregoing, it is a first aspect of the invention toprovide an improved wind turbine coupling with an asymmetric torsionalbehavior in wind turbine drive systems to protect gearbox bearings fromdamage due to torsional reversals.

Still a further aspect of the invention is the provision of a windturbine coupling system having very little wind-up or torsionaldisplacement during normal operation in the forward direction and yethaving significant torsional wind-up and/or displacement in the reversedirection.

Yet another aspect of the invention is the provision of a wind turbinecoupling system that is readily adaptable to existing wind turbinecouplings for enhanced operation and durability.

Another aspect of the invention is the provision in a wind turbinecoupling system of a frictional slip capability in parallel withtorsional wind-up and/or displacement.

Another aspect of the invention is the provision of a wind turbinecoupling system having a frictional slip capability high enough thatthere is little or no slippage during normal operation.

Yet a further aspect of this invention is the provision of a windturbine coupling system with a small amount of frictional slippageduring startup to full rated torque and shutdown.

Another aspect of the invention is the provision of a wind turbinecoupling having enough frictional slippage during a torque reversal toabsorb the damaging energy of an impact load that might otherwiseinitiate axial cracking in gearbox bearings.

Yet another aspect of the invention is the provision of a wind turbinecoupling that automatically resets itself to the forward operationalposition upon restart of the turbine to full torque.

Still a further aspect of the invention is the provision of a windturbine coupling of a symmetrical design that is able to operate in anasymmetrical manner described above so that the device can be used forboth clockwise and counter-clockwise forward direction of the windturbine generator.

The foregoing and other aspects of various embodiments of the invention,taken separately or in combination, are achieved by the improvement in awind turbine power generating system comprising a wind turbine connectedto a speed-increasing gearbox having a high-speed output shaft and anelectrical generator having an input shaft, the improvement comprising:a coupling system interconnecting said output and input shafts, saidcoupling system having a portion of its torque path split between atleast one of a torsional wind-up and displacement component along with atorsional dampening component. The torsional dampening component may bea frictional slip component set in the forward direction to at least 10%of the rated turbine torque. Similarly, the frictional slip setting inthe reverse driving direction may be equal to at least 10% of the ratedturbine torque. The torsional rotational displacement movement in areverse direction may be taken from the group of at least 10 degrees orgreater for turbines with generators operating at 1000 rpm or more, andfor turbines with lower generated speeds in the range of 1 to 5 degreesper 100 rpm. A zero backlash connection may be provided between apressure plate and endplate in the coupling system.

In a preferred embodiment of the invention for generator speedsexceeding 1000 rpm, the parallel wind-up and frictional slip are locatedon the generator shaft and an adapter plate is provided to fit theexisting flexible coupling of the turbine for ease and safety ofretrofitting. According to the invention, a portion of the rotationalmovement is frictional slip without torsional wind-up in forward orreverse and wherein the frictional slip-only portion is at least 10degrees. It is contemplated that a hard stop is provided after athreshold of torque in a forward direction is exceeded, and thatthreshold is contemplated at above 120% of rated turbine torque.

The torsional movement of the coupling system has an asymmetric actionduring operation such that the only time a deadband is engaged is duringa torque reversal exceeding a frictional slip threshold. It isparticularly preferred that the asymmetric action occurs automaticallyand a frictional slip setting is greater than 20% of the rated turbinetorque, but less than 100% thereof. The design of the frictional slipand wind-up elements are preferably symmetric to allow for use on windturbines that have gearbox designs that may drive the generator ineither clockwise or counterclockwise direction in normal forwardrotation.

In operation, the coupling system is symmetrical in design and yetasymmetric in its response to torque reversals with a prolonged slipability in either direction of shaft rotation, and the slip torquesetting is high enough to only occur during transient torsional eventsthat would typically include torque reversals and transient torqueevents. The torsional movement during normal startups and shutdowns isnormally less than 10 degrees for generator speeds exceeding 1000 rpm,while the torsional movement during transient torque reversals exceeds10 degrees. While a torsional spring is preferred for the wind-upaction, it is also contemplated that the torsional spring may bereplaced with an elastomeric material in shear, an elastomeric materialin compression, metal springs in compression, metal springs in bending,and gas springs. In any event, it is desired that the same torsionalwind-up components provide wind-up in both forward and reversedirections.

The invention also contemplates a method of providing torsional dampingin a wind turbine drive system to reduce the magnitude and rapidity oftorque reversals, and mitigate the resulting damaging impact loads ondrive system components, comprising: detecting a drive system torquereversal exceeding a first preset threshold; dissipating torsionalwind-up energy in the drive system while maintaining said reverse torqueat said first preset threshold; detecting a positive torque exceeding asecond preset threshold; and returning the turbine drive system toforward operation. The drive system operates in a forward directionproducing electric power without affecting the system's basic forwardtorsional characteristics, while providing torsional damping in areverse direction, and the referenced first threshold is set at lessthan 100% of the turbine torque at a power rating of the generator.Again, the torque reversal detection and dissipation of torsionalwind-up is achieved automatically with frictional slipping, whicheffectively reduces the magnitude of reverse torque and slows the rateof the torque reversal magnitude increase.

In certain embodiments of the invention, the first and second presetthresholds are the same and the frictional slipping action is providedin parallel with torsional springs that deflect during normal forwardoperation such that the torque load and the turbine generator drivesystem is shared by both frictional slippage and spring deflection. Thefrictional slippage provides a hysteresis damping to a winding up andunwinding of the drive system components, the torsional springspreferably having a zero torque load deadband for at least a portion ofa torsional displacement movement during a torque reversal and whereinadditional reverse torsional deflection spring action occurs at an endof the deadband movement. There is provided an additional reversetorsional spring action that is symmetric to the forward torsionalspring action, thus achieving bidirectional operation of the unit.

The invention also contemplates a method of retrofitting a wind turbinegenerator with torsional damping to reduce the magnitude and rapidity oftorque reversals, comprising: removing the coupling hub on the generatorshaft; installing a new coupling hub allowing the wind turbine generatordrive system to operate in the forward direction producing electricpower without affecting the system's basic forward torsionalcharacteristics while providing torsional damping in the reversedirection by detecting a drive system torque reversal exceeding a presetthreshold, dissipating torsional wind-up energy in the system whilemaintaining said reverse torque at said preset threshold, detecting apositive torque, and returning the turbine drive system to the forwardoperation; and selecting and installing an adapter plate to mate anexisting coupling spacer.

DESCRIPTION OF DRAWINGS

For a complete understanding of the various aspects, structures andoperation of the invention, reference should be made to the followingdetailed description and accompanying drawings wherein:

FIG. 1 is a functional schematic of a wind turbine coupling systembetween the gearbox and the generator particularly adapted as a retrofitfor existing coupling systems;

FIG. 2 is a graph of drive coupling torque as a percent of rated torquevs. angle of displacement for the present invention compared to thetorsional rigidity of the typical existing coupling hub it replaces,indicated by the vertical dot-dash line, during normal operation;

FIG. 3 is the same torque vs. angle of displacement graph as in FIG. 2,but shows the torque and displacement during a torque reversal andrestart of the turbine, the torsional rigidity and zero backlash ofexisting coupling systems being shown with the dot-dash line;

FIG. 4 is the same torque vs. angle of displacement graph as in FIGS. 2and 3, but if the turbine were turning in the opposite direction offorward rotation, e.g. counter-clockwise vs. clockwise, demonstrating asymmetric design with asymmetric behavior;

FIG. 5 is a cross-sectional view of the coupling adapted forimplementation of the system;

FIG. 6 is a cross sectional view of the coupling showing one of thecompression springs that provides the torsional deflection;

FIG. 7 is a sectional view of a coupling of the invention showing thecompression springs for the torsional deflection, along with the slotsfor the torque bolts in the friction plate, and the Bellville springsfor controlling the force on the friction elements;

FIG. 8 is a view of the input hub showing the holes for the torque boltsand the slots to allow rotational travel of torsional wind-upcompression springs without compression, along with the ends of thespring slots that provide for compression of the torsional wind-upsprings at each end of travel;

FIG. 9 is a view of the end plate showing the torque bolt holes, alongwith slots for movement and compression of the torsional wind-up springsand recesses for a Bellville spring;

FIG. 10 shows the friction plate with bonded friction material andopenings for the compression-type spring that is used for the torsionalwind-up, along with slots to allow the torque bolts to displacerotationally relative to the friction plate; and

FIG. 11 is the pressure plate that fits into the endplate with zerobacklash drive to transfer the torque between the two.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, and more particularly FIG. 1, it can beseen that a wind turbine coupling system between the gearbox outputshaft and generator input shaft made in accordance with the prior art isdesignated generally by the numeral 10. The system includes a gearbox 12coupled to the generator 19 by a spacer coupling 14. The instantinvention 16 replaces the existing coupling hub on the generator shaftand adapts to the spacer of the existing coupling system. The drawingillustratively shows a frictional slip capability 17 in parallel with atorsional deflection and/or displacement capability 18.

With reference to FIG. 2, an appreciation can be obtained as to thetorsional behavior of an existing wind turbine coupling hub compared tothe torsional behavior of a preferred embodiment of the invention. Theexisting coupling hubs are characterized by torsional rigidity and zerobacklash with virtually no useful torsional wind-up or angulardisplacement as shown with the near-vertical dot-dash line. Incontradistinction, the preferred embodiment has a useful level oftorsional wind-up (torque increasing with angular displacement) and/orsome significant torsional displacement. In parallel with the torsionalwind-up and/or displacement is a frictional slip capability preferablyset to allow some slight torsional displacement during startup andshutdown. A slip setting of 40% of rated turbine torque is shown toallow this slight frictional slippage during startup and shutdown andthus keeping the frictional interface in optimum condition. FIG. 2 alsoshows a potential for large angular displacement, but it does not becomeactive until a torque reversal, as shown in FIG. 3. Those skilled in theart can appreciate that a frictional slip setting that was much greaterthan 50% of rated turbine torque could prevent slippage in the forwarddirection. It could still function in the torsional reversal mode, whichis a rare event in typical modern turbines and thus the frictioninterface would have to be well protected from corrosion and otherfrictional interface changes that could cause stick/slip behavior or anundesirable increase in its static torque slip release setting.

FIG. 3 shows the behavior of the preferred embodiment during a torquereversal exceeding the frictional torque setting threshold. It shows theeffect of providing a large amount of angular displacement or backlashin conjunction with the torsional wind-up ability that is acting inparallel with the frictional slip ability. Torque reversals are known tocause load zone reversals on the gearbox bearings that result in therollers impacting on the bearing raceways. This can lead to cracking andfailing of the bearing races. By designing in a significant angulardisplacement, the frictional slippage can absorb most or all of theimpact energy that could cause bearing damage. Most torque reversalsoccur during severe stopping events. FIG. 3 shows that upon restartingthe turbine, a preferred embodiment automatically slips back into normaloperation when the forward torque in the system exceeds the frictionalslip threshold. The preferred embodiment also has a torsional wind-upability at the end of the reverse angular displacement. This providescushioning in case the amount of angular displacement designed is notenough to fully absorb the torque reversal energy. Some hard stops mayalso be designed in to limit the torsional wind-up and protect thetorsional wind-up springs from damage.

FIG. 4 shows another reason to provide the torsional wind-up ability inreverse—the symmetry of the design. FIG. 4 is again the same FIGS. 2 and3, but shows another advantage of designing the torsional wind-upability into the reverse direction—the design becoming symmetrical sothat the unit can operate the same whether the normal direction ofrotation of the generator shaft is clockwise or counterclockwise. Thiseliminates the need to manufacture and stock two different units for thesame size wind turbine that has different directions of rotation at thegenerator.

Referring now to FIG. 5, an appreciation of the invention can beobtained from a sectional view that shows the details of the preferredembodiment. Adapter 20 connects the coupling spacer 14 to the input hub22. Torque bolts 24 clamp the endplate 26 to the input hub 22 withspacing controlled by the bolt spacers 25. The endplate 26 retains theBelleville springs 32 that provide the necessary force on the pressureplate 28 against the friction material 30 to control the slip torquesetting. The friction material is affixed to the friction plate 34,which is fixed to the output hub 40 with bolts 38. Bearing 36 maintainsconcentricity of the input to the output. A “shrink disc” shaft lockingdevice 42 clamps the output to the generator shaft.

FIG. 6 is a sectional view similar to FIG. 5 that shows one of aplurality of compression springs 50 that provides a means for torsionalwind-up. FIG. 7 is a sectional end view showing one of the compressionsprings 50 compressed in the normal operating position with the torquebolts 24 near the end of travel in the slots 52 in frictional plate 34.The slot ends provide a hard stop against the torque bolts to protectthe compression springs from being overstressed. A plurality ofBelleville springs 32 is also shown. These provide the necessary axialforce for the characteristic slip torque on the friction surfaces 30.

FIG. 8 shows the details of the input hub 22 with holes 58 toaccommodate the torque bolts 24. Slots 54 provide clearance for thecompression springs 50 to allow rotational movement between the inputand the output when the frictional torque threshold set by theBelleville springs 32 is exceeded. Slot ends 56 contact the compressionsprings 50 at the extremes of the angular displacement to provide thedesired torsional wind-up capabilities at each end of the torsionaldisplacement.

FIG. 9 is a view of the endplate 26 showing slots 60 matching the slots54 for the rotational movement relative to the compression springs.Similar slot ends are also shown. Recesses 62 for the Belleville springsare also shown.

FIG. 10 is a view of the friction plate 34 showing slots 66 for relativemovement of the torque bolts and openings 68 for containing thecompression springs 50.

FIG. 11 is a view of the pressure plate 28 with tabs 70 shown as a meansof carrying the torque to the endplate.

It is contemplated that various embodiments will typically have acombination of torsional wind-up and torsional displacement that willexceed 10 degrees and preferably be on the order of 20 to 60 degrees orgreater for typical turbines with generators operating at 1000 rpm ormore. For turbines with lower generator speeds, the required torsionaldisplacement would be lower, in the range of 1 to 5 degrees per 100 rpm.

It is also contemplated that various embodiments will typically have africtional torque setting exceeding 10% of the turbine torque so thatnormal turbine torque fluctuations do not cause unnecessary slippage andwear. The frictional torque setting would preferably be in the range of20 to 50%, but could also be in excess of 50%. The most preferredsetting would be 30 to 45% so as to provide a slight amount of slippageduring normal startup and shutdown. That keeps the friction interface atits optimum performance during the rare torque reversal events that candamage the bearings.

Thus, it can be seen that the various aspects of the invention have beenachieved by the structure presented and described above. While inaccordance with the patent statutes, only the best known and preferredembodiment of the invention has been presented and described in detail,the invention is not limited thereto or thereby.

Accordingly, for an appreciation of the true scope and breadth of theinvention, reference should be made to the following claims.

What is claimed is:
 1. A method of providing torsional damping in a windturbine drive system for a generator to reduce the magnitude andrapidity of torque reversals, and mitigate the resulting damaging impactloads on wind turbine drive system components, comprising: detecting awind turbine drive system torque reversal exceeding a first presetthreshold; dissipating torsional wind-up energy in the wind turbinedrive system while maintaining said reverse torque at said first presetthreshold; detecting a positive torque exceeding a second presetthreshold; returning the wind turbine drive system to forward operation;wherein detecting a wind turbine drive system torque reversal anddissipating torsional wind-up energy are achieved automatically byfrictional slipping and wherein an angle of torsional displacement ofsaid wind turbine drive system is sufficient to cause said frictionalslipping to effectively reduce the magnitude of reverse torque and slowa rate of torque reversal magnitude increase, and wherein said generatoroperates at speeds greater than 1000 rpm and said angle of torsionaldisplacement exceeds 10 degrees.
 2. The method as recited in claim 1,which allows said wind turbine drive system to operate in a forwarddirection producing electric power through said generator withoutaffecting the system's forward operation, while providing torsionaldamping in a reverse direction.
 3. The method as recited in claim 2,wherein said first preset threshold is set at less than 100% of aturbine torque at a power rating of the generator.
 4. The method asrecited in claim 1, wherein said first and second preset thresholds arethe same.
 5. The method as recited in claim 1, wherein said frictionalslipping is in parallel with torsional springs that deflect duringnormal forward operation such that a torque load in the turbinegenerator drive system is shared by both frictional slippage and springdeflection.
 6. The method as recited in claim 5, wherein said torsionalsprings have a zero torque load deadband for at least a portion of atorsional displacement movement during a torque reversal.
 7. A method ofproviding torsional damping in a wind turbine drive system for agenerator to reduce the magnitude and rapidity of torque reversals, andmitigate the resulting damaging impact loads on wind turbine drivesystem components, comprising: detecting a drive system torque reversalexceeding a first preset threshold; dissipating torsional wind-up energyin the wind turbine drive system while maintaining said reverse torqueat said first preset threshold; detecting a positive torque exceeding asecond preset threshold; returning the turbine drive system to forwardoperation; and wherein said first preset threshold is set at less than100% of the turbine torque at a power rating of the generator, whereindetecting a wind turbine drive system torque reversal and dissipatingtorsional wind-up energy are achieved automatically by frictionalslipping, and said generator operates at speeds under 1000 rpm, and saidangle of torsional displacement exceeds 1 degree per 100 rpm.
 8. Themethod as recited in claim 7, wherein a frictional slipping threshold isset at between 20% and 80% of rated turbine operating torque.
 9. Amethod of providing torsional damping in a wind turbine drive system fora generator to reduce the magnitude and rapidity of torque reversals,and mitigate the resulting damaging impact loads on wind turbine drivesystem components, comprising: detecting a wind turbine drive systemtorque reversal exceeding a first preset threshold; dissipatingtorsional wind-up energy in the drive system while maintaining saidreverse torque at said first preset threshold; detecting a positivetorque exceeding a second preset threshold; returning the turbine drivesystem to forward operation; wherein detecting a wind turbine drivesystem torque reversal and dissipating torsional wind-up energy areachieved automatically by frictional slipping and wherein saidfrictional slipping is in parallel with torsional springs that deflectduring normal forward operation such that a torque load in the turbinegenerator drive system is shared by both torsional spring deflection andsaid frictional slipping, and said torsional springs have a zero torqueload deadband for at least a portion of a torsional displacementmovement during a torque reversal.
 10. The method as recited in claim 9,wherein the frictional slipping provides hysteresis damping to a windingup and unwinding of the wind turbine drive system components.
 11. Themethod as recited in claim 9, wherein said generator operates at speedsabove 1000 rpm and said zero torque deadband of the torsionaldisplacement exceeds 10 degrees.
 12. The method as recited in claim 9,wherein said generator operates at speeds below 1000 rpm and said zerotorque deadband of the torsional displacement exceeds 1 degree per 100rpm.
 13. The method as recited in claim 9, wherein a reverse torsionalspring deflection action occurs at an end of the deadband movement. 14.The method as recited in claim 13, wherein said reverse torsional springdeflection action is symmetric to a forward torsional spring deflectionaction thus achieving bidirectional operation of said wind turbine drivesystem.
 15. The method as recited in claim 14, wherein a total torsionalspring deflection action of said forward and reverse torsional springdeflection actions and zero torque deadband exceeds 10 degrees for windturbines with generator operating speed exceeding 1000 rpm.
 16. Themethod as recited in claim 14, wherein said total torsional springdeflection and zero torque action deadband exceeds 1 degree per 100 rpmfor wind turbines with said generator operating speed under 1000 rpm.